Conventionally, thermal energy of exhaust gas exhausted from automobile engines has been recovered by employing techniques. For example, there has been proposed an exhaust heat recovery system attaching an exhaust heat recovery device (e.g., a heat exchanger) to an automobile's exhaust heat system to absorb thermal energy of exhaust gas to use the absorbed thermal energy for heating, a water heater, and the like. In The thus configured exhaust heat recovery system if the exhaust heat recovery device has abnormality the abnormality must be detected as early as possible in order to prevent secondary abnormality attributed to the exhaust heat recovery device. For example, Japanese Utility Model Laying-Open No. 61-58502 discloses an exhaust gas heat exchanger damage detection device capable of preventing an engine from damage caused when the exhaust gas heat exchanger is damaged. This damage detection device provides at an intermediate portion of an exhaust pipe connected to the engine the exhaust gas heat exchanger to recover thermal energy of exhaust gas in the form of hot water and vapor. The damage detection device is provided with an abnormality detector detecting variation in temperature or pressure caused at locations closer to a water inlet and a water outlet, respectively, that is caused when the exhaust gas heat exchanger is damaged. When these abnormality detectors detect abnormality and signal accordingly, the damage detection device responsively closes an electromagnetic valve provided at an intermediate portion of the exhaust pipe connecting the engine and the exhaust gas heat exchanger or an electromagnetic valve provided at the exhaust gas heat exchanger adjacent to the water inlet.
As disclosed in the document, the damage detection device includes an abnormality detector provided at an exhaust gas heat exchanger adjacent to a water inlet and a water outlet to detect whether the exhaust gas heat exchanger has abnormality and if so the damage detection device closes an electromagnetic valve provided at an exhaust pipe or the exhaust gas heat exchanger adjacent to the water outlet to prevent an engine from damage caused when the exhaust gas heat exchanger is damaged.
An exhaust heat recovery system having an exhaust heat recovery device connected parallel to an exhaust pipe is provided with a switching valve switched to pass exhaust gas from the exhaust pipe to the exhaust heat recovery device in recovering exhaust heat. If the switching valve has abnormality the exhaust heat recovery device cannot receive the exhaust gas and hence recover thermal energy.
The damage detection device disclosed in the aforementioned document has the exhaust heat recovery device provided in series with the exhaust pipe, and does not allow for the switching valve's abnormality.